Free radical vs. electron-transfer routes of oxidation of hydrocarbons by laccase/mediator systems. Catalytic or stoichiometric procedures

The oxidation of C–H bonds in alkylarenes can take place by dioxygen, under catalysis by the phenol-oxidase enzyme laccase, provided that suitable mediator compounds are added. The actual oxidation of the substrate is carried out by the oxidised form of the mediator, in a non-enzymatic step. The relative efficiency of four >N–OH-type mediators (HBT, HPI, VLA, NHA) has been evaluated, and compared with that of the structurally different mediator ABTS. Laccase/mediator catalysed oxidations of non-phenolic substrates can proceed via two different mechanisms. Either on monoelectronic oxidation, by the oxidised form of mediator ABTS, or, by abstraction of hydrogen atom, by a >N–O• radical species derived from the >N–OH-type mediators. The former mechanism requires substrates with a low oxidation potential; the latter mechanism requires substrates with relatively weak C–H bonds. Electrochemical and thermochemical evidence is provided (i) to explain the failure in the oxidation of specific alkylarenes and (ii) in support to the rationalisation of the experimental findings. Particular emphasis is given to discuss the effect of the mediator-to-substrate molar ratio upon the efficiency of the oxidation procedure. In order to explain why, in previous literature studies, better results may have been obtained by using the mediator in more that stoichiometric amounts, we propose the concurrent formation of degradation products from the mediator, which could be responsible for the onset of alternative oxidation pathways. A better understanding of the natural role of laccase in the oxygen-dependent degradation of lignin in wood emerges from this study.